Lumber which has recently been cut and machined contains a relatively large percentage of water and is referred to as green lumber. Prior to being used in construction or other applications which demand good grades of lumber, the green lumber must be dried to remove a relatively large percentage of water from the lumber. Acceptable water content will vary with the application as well as the type of wood, however, in many circumstances, a moisture content of 19% or less is an acceptable water content.
Although lumber may be air dried, kiln drying accelerates and provides increased control over the drying process. In kiln drying, a charge of lumber is placed in a kiln chamber. A typical kiln chamber is a generally chambers typically have reversible fans for circulating the air through the chamber.
The charge of lumber placed in the kiln generally consists of a number of rectangular solid stacks of lumber. Each stack of lumber, in turn, typically consists of a number of vertically stacked, horizontal rows of lumber that form a rectangular solid. The horizontal rows are spaced apart for air to pass between the rows using wooden boards referred to as "stickers" that have a relatively small lateral cross-sectional area in relation to the lateral cross-sectional area of the lumber forming the charge. The stickers are generally spaced apart between each horizontal row to allow air to flow between the rows.
Typically, the stacks of lumber are placed on separate wheeled, flat bed cars which are mounted for movement on railroad-type tracks. Kilns may have any desired number of tracks. Multi-track kilns may therefore accept several stacks of lumber during each drying cycle.
In operation, a charge of green lumber is initially placed in a kiln chamber. After sealing the kiln chamber, the air within the kiln is heated to facilitate drying. The air may be heated in a number of ways such as by heat transfer from pipes extending through the kiln chamber in which steam flows. Alternatively, heated air may be introduced such as from a furnace. Kilns which utilize the introduction of heated air are typically referred to as direct fired kilns.
Fans generally positioned in upper portions of the kiln and above the stacked lumber circulate the heated air through the kiln chamber, including the stacks of lumber. Because the stickers provide spacing between the horizontal rows of lumber, the heated air passes between the rows of lumber and is in direct contact with both the upper and lower surfaces of the individual pieces of lumber. The fans continually recirculate the air through the kiln and the lumber to further dry the lumber. Periodically, a portion of the circulating air is exhausted from the kiln and additional air is introduced into the kiln. The additional air is typically heated in the kiln chamber, such as by heat transfer from the steam pipes. This periodic exhaust and replacement process allows circulating air which has absorbed a large amount of moisture from the green lumber to be removed, while drier air is introduced to accelerate the lumber's drying.
Within such kilns, the circulating air flows in a generally circular pattern. More particularly, fans above the stacks of lumber direct air laterally over the top of the lumber in a first direction. When the air contacts a first sidewall of the kiln chamber, the bulk of the air is forced downward by the fans and the ceiling of the kiln chamber. The circulating air subsequently flows through the spaces between the horizontal rows of lumber established by the stickers in a second, lateral direction opposite the first lateral direction. Upon contact with a second sidewall of the kiln chamber, the air rises and is recirculated by the fans through the lumber. Periodically, the fans are reversed such that the air flows in the opposite direction to provide generally consistent drying of the lumber.
Kiln chambers are generally rectangular in lateral cross-section. Further, the stacks of lumber are generally rectangular in lateral cross-section since they are comprised of a number of vertically stacked, horizontal rows of substantially equal width. Accordingly, the airflow within the kiln chamber does not proceed smoothly in the circulation path established by the fan. Instead, turbulence is introduced by, among other things, the sharp corners and flat walls of the kiln chamber and the sharp corners of the stacks of lumber.
In particular, the upper corners of the stacks of lumber about which the circulating air must pass creates significant turbulence and may establish localized eddy currents. While in some instances air turbulence is beneficial, the turbulence created by the upper corner of the stacks of lumber and the establishment of eddy currents prevents the smooth circulation of air to and from the fans and decreases the drying efficiency of the lumber since less air flows over the surface of the lumber decreasing the drying capacity of the kiln. Since it is expensive and time consuming to heat the air to dry the lumber, the resultant increase in drying time significantly increases the expense of kiln drying.